1. Field of the Invention
The present invention relates to a core catcher.
2. Description of the Related Art
If supply of water into a reactor pressure vessel is stopped or cooling water is lost due to a rupture in piping connected to the reactor pressure vessel in a water-cooled type reactor, a reactor water level drops to expose a reactor core (which will be simply referred to as “core”, hereinafter), which may result in insufficient cooling of the core. In anticipation of such a case, the idea of automatically bringing the reactor to emergency shutdown upon receiving a signal of low water level and submerging and cooling the core by an ECCS (emergency core cooling system) injecting a coolant has been adopted to prevent a meltdown accident.
However, though a situation in which the emergency core cooling system described above does not work and other devices for injecting water into the core are not available is extremely unlikely to occur, it may still occur. In such a situation, a drawdown in the reactor water level exposes the core, and the core fails to be sufficiently cooled. Decay heat continuously generated even after shutdown of the reactor raises fuel rod temperature, which may lead ultimately to core meltdown.
In such a case, high-temperature core molten material (which may be called “corium”) falls onto a lower portion of the reactor pressure vessel, melts through a reactor pressure vessel lower head, and falls onto a floor inside a containment vessel. The corium heats concrete covering the containment vessel floor. When a surface in contact with the concrete becomes hot, the corium reacts with the concrete to generate a large quantity of non-condensable gas containing, e.g., carbon dioxide and hydrogen and to melt and ablate the concrete. The generated non-condensable gas may pressurize the containment vessel and break the reactor containment vessel. The melting and ablation of the concrete may break a containment vessel boundary and decrease structural strength of the containment vessel. If the reaction between the corium and the concrete continues, the containment vessel may break to emit a radioactive material in the containment vessel into the external environment.
In order to inhibit the reaction between the corium and the concrete, it is necessary to cool a surface of a bottom of the corium which is in contact with the concrete to the melt temperature or lower (1500 K or less for typical concretes) or to prevent the corium from coming into direct contact with the concrete. Therefore, various countermeasures are proposed against falling of the corium, as disclosed in, e.g., Patent Documents 1 to 4 (Japanese Patent No. 3510670, Japanese Patent No. 3150451, Japanese Patent Laid-Open No. 2007-225356 (JP-A 2007-225356), and Japanese Patent No. 3424932).
There is a growing need for measures to hold a molten core, at home and abroad. The growing need for measures holds true not only for reactors to be newly built but also for existing reactors. Particularly in existing reactors, safety is ensured by accident management. It is thus desired that hardware remedies for severe accidents, such as installation of a corium holding structure, be applied also to existing reactors.
However, fabrication (installation) of a structure for holding a molten core such as a core catcher (which will be referred to as “corium holding structure”, hereinafter) as a measure to hold a molten core by application of conventional techniques as disclosed in Patent Documents described above is not necessarily easy. This applies not only to existing reactors not fabricated on the premise of installment of a corium holding structure but also to reactors to be newly built.
If a corium holding structure is installed in a newly built reactor by applying conventional techniques, it is necessary to lay a heat resisting material in order to thermally protect the structure for holding a molten core and provide a cooling unit for supplying cooling water in order to suppress thermal ablation of the heat resisting material. The heat resisting material needs to be laid after piping or the like serving as a cooling passage is installed, and there are installment constraints such as a need for installation space including space for the cooling passage. Installment of a corium holding structure is thus not necessarily easy.
If a corium holding structure is installed in an existing reactor by applying conventional techniques, the existing reactor not fabricated on the premise of installment of a corium holding structure has no installation space or the existing reactor has enough space but has constraints on installation methods. An existing reactor has more installment constraints than a newly built reactor. Installment of a corium holding structure is thus not necessarily easy.
The idea of installing a corium holding structure without a cooling unit under a heat resisting material is also available in order to facilitate installment of a corium holding structure in a newly built reactor. The idea requires use of an expensive heat resisting material with high heat resistance which can withstand thermal ablation by high-temperature corium, which results in a cost increase for the corium holding structure.